Sensor arrangement and method in digital x-ray imaging

ABSTRACT

The invention relates to solutions where biased (photoelectric) (semiconductor) material (e.g. Ge, Si, Se, GaAs, HgI, CdTe, CdZnTe, PbI) converts X-ray quanta directly into electron-hole pairs, each of which, by using an intensive electric field connected across said material, can be collected, avoiding lateral diffusion, into the area of its respective pixel. The X-ray quanta are detected and the amount of the ones exceeding the possibly adjustable threshold level is counted. According to the invention the reading electronics ( 20 ) may be implemented by providing for each of the pixels ( 31, 31 ′, . . . ) its own junction surface ( 32 ) which is connected to the said semiconductor material in a manner by which the reading out of them, as well as for enabling the TDI imaging, the counters ( 23 ′, . . . ) for each of the pixels ( 31 ′, . . . ) can be loaded parallel from the counters ( 23, 23 ′, . . . ) for pixels on the same row in the preceding pixel column.

[0001] The present invention relates to a sensor arrangement as definedin the preamble of claim 1 and to a method in digital X-ray imaging asdefined in the preamble of claim 17.

[0002] About ten years ago, digital X-ray imaging emerged from researchunits and began to gain ground as a practical imaging method. Ignoring afew exceptions, the earliest digitizing methods were based on aprocedure in which X-ray quanta having penetrated the object beingimaged were absorbed into a so-called scintillator, which consequentlyemitted photons, i.e. in a way converted the energy level of the X-rayquanta to a wavelength of light. The photons were subsequentlytransferred either directly or generally via an optical medium onto asilicon substrate, in which, on being absorbed into the substrate, thephotons formed electron-hole pairs, i.e. charges capable of beingdetected by electrical means. In regard of efficiency, however, sucharrangements were relatively modest, and in regard of resolution theywere poor because their principle involved the problem of diffusion ofphotons in the scintillator. On the other hand, as the thickness of thescintillator layer therefore had to be kept rather small, typically atabout 10 μm, another consequence was that a large proportion of theX-ray quanta penetrated the scintillator layer and was absorbed into theoptical medium. It could even happen that x-ray quanta were absorbedinto the silicon substrate itself, which produced a high level ofquantum noise in the image information. Often only about 30% of theX-ray quanta could be utilized in image formation.

[0003] With the development of the technology, arrangements as describedabove have yielded a quantum efficiency (dqe) of over 50%. However, theimaging resolution typically achieved by these type of solutions isstill only about 10 lp/mm, after which the MTF (Modulation TransferFunction) describing the resolution begins to fall rapidly, being at alevel of only about 30% e.g. at frequencies exceeding 5 lp/mm.

[0004] In the traditional scintillator material, an X-ray quantumproduces, depending on its energy, about 20 photons/keV in the directionof the silicon substrate used as a detector. A proportion of the photonsdisappears during the passage through the optical fiber transfer medium.Depending on the magnification produced with the fiber optics, typically10-70% of the photons disappear, which means that even in a favorablecase only about 18 photons/kev can be received at the detector. Using aCCD (charge-Coupled Device) sensor having a light quantum efficiency ofabout 0.3, approximately 6 electrons/keV can be detected. Thus, withX-radiation with an energy level of e.g. 20 keV, the final signal thatcan be detected is only about 100 electrons/quantum. When such atechnique is used, special care has to be taken in designing the sensorelectronics to ensure that no more of the information carried by thequanta having penetrated the object gets lost and that every electrondetected gets measured.

[0005] In the foregoing, reference was made to a CCD sensor, which is adetector generally used in these type of sensor systems. However, aproblem essentially associated with CCD technology is the presence ofso-called dark current, which arises from the sensor's own surface creepcurrent, which in the course of time leads to an accumulation of signalin the so-called pixel wells of the CCD sensor. Therefore, even if noradiation falls on the sensor, the dark current produces background fogin the image obtained from the sensor, which is a significantdisadvantage especially at low signal levels.

[0006] Another typical problem with the CCD technique is overexposure.If there is too much signal energy, the charges will start flowing outfrom the pixel wells into neighboring pixels. This messes up the imageand, at the charge transfer phase of the CCD technique, producesso-called blooming.

[0007] In all medical X-ray imaging the aim is to keep the dose ofradiation the patient is exposed to as low as possible withoutcompromising on image quality. One of the essential factors in thisrespect is to obtain a quantum efficiency as high as possible,preferably so that all the X-ray quanta having penetrated the target canbe made to contribute to image formation. On the other hand, in respectof image quality, it is in many applications important to achieve amaximal imaging resolution. E.g. in mammography, the detection of smallmicro-calcifications of a size below 100 μm is extremely important.

[0008] Technological development has led to a new X-ray quanta detectiontechnique in which the low-efficiency light conversion step is left outaltogether. In such a technique, the X-ray quanta are absorbed into amedium (e.g. Ge, Si, Se, GaAs, HgI, CdTe, CdZnTe, PbI) in which they aredirectly converted into electron-hole pairs. When an intensive electricfield is applied across such a medium, the charges produced by theabsorption of X-ray quanta can be directed toward pixel electrodes in away that their lateral movement toward adjacent pixels is in practiceprevented. Such a technique provides the advantage of allowing thethickness of the X-ray quanta absorbing layer to be increased up to atheoretical quantum efficiency of 100% without any substantial loss ofimaging resolution. To achieve such a level of efficiency, e.g. asilicon layer having a thickness of the order of about 3 mm would beneeded, but when e.g. ZdZnTe is used, a thickness of about 0.5 mm willbe sufficient for the recovery of all X-ray quanta at 20 keV energylevel.

[0009] However, the new technology as described above also has certaindrawbacks which lead to some limitations regarding the possibilities ofutilization of this otherwise excellent technique. As the X-ray quantaabsorbing layer in any case has to be relatively thick, it must bearranged in a position as perpendicular as possible to the X-ray beam toensure that the absorption of quanta at different depths will not leadto their being imaged in the areas of different pixels and therefore toa degradation of lateral imaging resolution. Thus, especially for thepurposes of medical X-ray imaging, constructing a so-called full-fieldsensor using this technology is a somewhat dubious enterprise becausethe resolution of such a sensor is considerably lower in the edge zonesthan in the central area. As this problem is common to all sensors basedon direct absorption that were known before the present invention, theirperformance has only been measured in the central part of the imagearea, where the best imaging result is obtained. By contrast, such atechnology would be excellently applicable in so-called narrow-beamscanning imaging, where a narrow sensor can be kept substantiallyperpendicular to the X-ray beam during the entire scanning process.However, to achieve a sufficient signal sensitivity, scanning imagingwould require a sensor structure of a width of several pixels and eitherthe utilization of so-called TDI (Time Delay Integration) technology ora signal reading speed that is unattainable by prior-art solutions inpresent-day technology.

[0010] In full-field sensors like those described above, the imageinformation is generally read using reading electronics provided on thesurface of an amorphous silicon substrate. Another possible solution isto use a sensor composed of smaller modules and to implement the readingelectronics using CMOS (Complementary Metal-Oxide Semiconductor)technology. In both of these techniques, the image information isgenerally read either after the imaging and sometimes also during it,pixel by pixel, by addressing one pixel at a time and reading the chargeaccumulated in it from the edge of the sensor. Especially in largesensors, however, there is the problem of the pixel charge beingdistributed into reading channel capacitances, producing noise in thesignal being measured, and, as stated above, even scanning imagingcannot be implemented because it is not possible to reach the requiredreading speed by means available today.

[0011] When X-ray quanta are converted directly into electron-holepairs, the signal produced is very large, about 200 keV, as comparedwith conversion into light. Thus, a 20 keV quantum as mentioned aboveproduces a charge of about 4000 electrons instead of 100 electrons,which means that the problems encountered by the sensor electronics arethe converse of the traditional problems. This is to say that the signalnow obtained is so large that its processing is becoming difficult. Toachieve a sufficient gray scale resolution, an information depth of atleast 12-14 bits would be required, which in this case would mean a needto process charges of at least 16-65 Me. Therefore, the use of e.g. CCDsensors in the reading electronics would be practically impossiblebecause the maximum charge they are able to transfer is only about 500000 e⁻. Because of the unsuitability of state-of-the-art readingelectronics, this type of detector, though especially suited forscanning imaging, is inapplicable for this purpose as the signal cannotbe collected even by the known TDI technique utilizing CCD sensors.

[0012] Another disadvantage impeding the use of direct conversionsensors is the limited propagation speed of charge elements in absorbingmaterials currently used, which gives rise to so-calledpost-luminescence, which in the worst case may continue for as long asseveral hours. To deal with this phenomenon, artificial compensation hasbeen employed by taking the information of previous images into accountand subtracting it as a function of time from the image taken last.However, this method cannot fully correct the error arising as a resultof trapped charges drifting with time even laterally into the area ofneighboring pixels.

[0013] Digital imaging methods used for medical purposes can be dividedinto two main categories referred to above, full-field imaging andscanning imaging performed using a narrow sensor. Considering thepractical imaging process, full-field imaging corresponds to traditionalimaging on a film the size of the entire image area. A distinct drawbackassociated with this technology is the need for large and therefore veryexpensive sensors and the need to eliminate the secondary radiationscattering from the object being imaged, requiring the use of complexmechanical grid arrangements. Because of their principle of operation,these grid arrangements also cause a doubling of the dose of radiationneeded for imaging.

[0014] The narrow sensor used in scanning technology requires somemechanical support, but the costs involved are still considerably lowerthan those for a full-field sensor. Moreover, scanning imaging requiresno grid, so the radiation dose applied to the object to be imaged iscorrespondingly halved. However, because of the small pixel size (highresolution) needed e.g. in mammography, it would still be necessary touse the TDI method and a sensor having a width of several pixels inorder to obtain a sufficient signal with an X-radiation output of apractical magnitude. In state-of-the-art solutions, TDI imaging hasgenerally been implemented using a CCD sensor for signal detection, butin an arrangement based on direct detection such a sensor would not, forthe reasons explained above, be capable of reasonable transfer of thesignal produced. On the other hand, another state-of-the-art methodwould be to read the signals detected by pixels connected to an X-Ymatrix one at a time by turns, but in the light of the scanning speedand resolution involved in the present applications, this would require12-bit A/D conversions and recording to be performed at a speed of about1 ns, which is beyond the capabilities of the technology availabletoday.

[0015] Consequently, the object of the present invention is to developnew solutions for improving the usability of sensors based on directdetection of X-radiation and especially to enable TDI imaging by using anew implementation of the technique of reading the detected signal. Thepresent invention also includes a possibility of monitoring the signalstrength during imaging in order to achieve an optimal exposure of theobject to be imaged.

[0016] The exact essential features of the invention are presented inthe claims below, especially in the characterizing parts of theindependent claims. A significant part of the invention consists of atechnology employed in devices known in themselves, such as Geigercounters, used e.g. in scientific physical research, according to whicheach quantum detected by the sensor is counted separately, specificallyfor the location and, if necessary, for the energy level as well. Inprior art, some ideas about applying an approach like this to medicalimaging as well are known, but the solutions so far presented aretechnically extremely difficult to implement and therefore inapplicableas a basis for a practical and cheap implementation of a sensor solutionsuited for digital X-ray imaging.

[0017] Thus, the present invention aims at utilizing the best aspects ofthe sub-areas of the technologies described above to disclose anefficient high-resolution sensor solution that is easy to manufactureand is therefore cheap. The invention combines the above-describedadvantages associated with different imaging methods into a single wholewhich, in its various embodiments, may comprise a sensor that, inprinciple, has a 100−% quantum efficiency and is capable of handling theentire signal generated and eliminating the effects of so-calledpost-luminescence and is immune to scattered radiation, dark current andoverexposure, eliminates thermal noise, is applicable for eitherfull-field imaging or scanning narrow-beam imaging, and is capable ofimage information transfer at the required speed and, when necessary,also of resolving the energy levels of the detected radiation, thus alsoallowing a more detailed analysis of the object being imaged.

[0018] In other words, the invention is based on solutions known inthemselves in which a biased (photoelectric) (semiconductor) material(e.g. Ge, Si, Se, GaAs, HgI, CdTe, CdZnTe, PbI) converts X-ray quantadirectly into electron-hole pairs, each of which, by using an intensiveelectric field connected across said material, can be collected,avoiding lateral diffusion, into the area of its respective pixel, thusmaking it possible to achieve 100% dqe, yet without compromising onresolution. According to the invention, however, the image informationproduced by the X-ray quanta having penetrated the object to be imagedis not detected on the basis of the (electron) charge generated in thearea of the pixel during imaging as in prior-art solutions but on thebasis of principles known in other areas of physics by computing thepure number of detected X-ray quanta exceeding a possibly adjustablethreshold level. By applying an adjustable threshold level, it ispossible to define a minimum energy level for the quanta to be counted,so that thermal noise appearing in the sensor and lower-energy radiationor the like produced by scattered radiation or post-luminescence can beeliminated directly at pixel level.

[0019] In the following, the invention will be described in detail bythe aid of its preferred embodiments and with reference to the attacheddrawings, wherein

[0020]FIG. 1 illustrates the structure of a typical sensor based ondirect detection of X-radiation,

[0021]FIG. 2 presents an electronic detection circuit suited for use inthe invention,

[0022]FIG. 3 presents a reading electronics solution according to theinvention,

[0023]FIG. 4 presents a reading electronics solution according to theinvention, considered at sensor level,

[0024]FIG. 5a and 5 b illustrate a modular sensor arrangement accordingto a preferred embodiment of the invention,

[0025]FIG. 6 presents a modular sensor arrangement according to a secondpreferred embodiment of the invention, and

[0026]FIG. 7 presents an electronics arrangement suited for use in theinvention, designed for adjustment of the minimum energy level of anX-ray quantum to be detected by a pixel.

[0027]FIG. 1 illustrates the basic structure of a typical sensor 10based on direct detection of X-radiation, wherein the element 11 usedfor absorbing X-radiation 12 is a material layer having an area of X×Yand converting the radiation directly into an electrical signal, saidmaterial layer being placed in an intensive electric field V. The layerconverting the radiation may consist of e.g. a relatively thin(semiconductor) material structure, (Ge, Si, Se, GaAs, HgI, CdTe,CdZnTe, PbI) with pixel electrodes arranged on the surface not visiblein FIG. 1, i.e. on the surface opposite to the surface facing toward theX-radiation 12, said pixel electrodes covering said surface in a desiredmanner. Thus, for each pixel, the electric field collimates the signalgenerated and the signal can be detected e.g. by reading electronics 20comprising a substrate having an area substantially equal to that of theabsorbing element 11 and by a spherical indium junction 13 connnected toeach pixel electrode. The reading electronics may be implemented usinge.g. CMOS technology.

[0028] At pixel level, the detection electronics 20′ comprised in thereading electronics 20 may comprise, as shown in FIG. 2, e.g. a signalamplifier 21 and a comparator 22, which either does or does not detectthe quantum absorbed into the pixel area, depending on the referencelevel V_(ref), which, if desired, may be externally adjustable. Inaddition, the solution presented in FIG. 2 comprises a preferably12-16-bit digital counter 23, which counts each voltage or current pulsewhose energy 35 level exceeds the reference level. The counter 23 can beprovided with a circuit for preventing counting after the counter hasreached its maximum value, ensuring that overexposure will not produceany other error in the image except that the pixel signal being measuredis at a maximum.

[0029] The reading electronics 20 of the invention may be implementede.g. as illustrated in FIG. 3 by providing for each pixel 31, 31′, . . .its own junction surface 32 and connecting the counters 23, 23′, . . .to each other in the direction of the imaging scanning movement of thesensor 10 so that, to allow the results of the counters 23, 23′, . . .to be read out in as simple a manner as possible and at the same time tomake TDI imaging possible, the counter 23′, . . . for each pixel 31′, .. . can be loaded in parallel from the counter 23, 23′, . . . for thepixel on the same row in the previous pixel column. The counters in thefirst column can be arranged to be reset to zero upon being loaded, sothe sensor signals can be easily reset.

[0030] As shown in FIG. 4, the reading electronics 20 of the sensor 10can be so implemented that the results of the outermost counters can beloaded in parallel into a shift register 41, in TDI imaging expressly ashift register placed on the side of the trailing edge of the sensor,shifting out the bits of one row in serial form in sequence. Thus, thesensor can be used for both full-field imaging and TDI imaging, with aminimum of only one output signal 42 being needed to output the imagedata produced by the sensor 10. Alternatively, serial data can be loadedinto the beginning of the shift register 41 e.g. from a precedingidentical sensor module 10 comprised in the same sensor arrangement, inwhich case the entire image information detected by the sensor systemconsisting of a number of separate modules can be read out via only onesignal line.

[0031] Especially in stationary imaging, by implementing the loading ofthe values of the outermost counters into the shift register 41 as anautonomous function independent of the other counters, it will bepossible to read out the values of this counter chain via the shiftregister 41 while the imaging and counting of quanta are simultaneouslygoing on all the time. Such an arrangement can be used for continuousmeasurement of the detected signal and adjustment of optimal exposure ofthe object to be imaged without losing the value already counted. Byloading the values of the outermost counters into the shift register 41simultaneously in connection with parallel loading of all counters ofthe sensor 10, the sensor can be used for reading out the image data inTDI imaging and full-field imaging in the manner described above. If asufficiently small pixel size has been selected for use as the physicalpixel size of the sensor, then it will be possible to combinepixel-specific signals read into larger entities e.g. in a computerarranged for image processing. The image signal can also be processed soas to increase the resolution of TDI imaging in the direction of themovement, making it possible to optimize the radiation dose/resolutionrequired in the case of each individual object to be imaged, e.g. asdescribed in FI patent 97665.

[0032] In the sensor of the invention, the lower-energy quanta producedby e.g. thermal noise or scattered radiation can be eliminated directlyat pixel level by using an arrangement in which the reference level forthe signal of each pixel column or preferably every second pixel columncan be externally adjusted. This makes it possible e.g. in TDI imagingto have adjacent pixels imaged with different energy level resolutionsby setting at each column transfer different reference levels betweencolumn pairs (or series), thus allowing one or more images adjusted todifferent energy levels to be obtained from almost the same point in theobject. Using such images, a more accurate analysis of the object imagedwill be possible.

[0033] The modular sensor arrangement of the invention can also beimplemented e.g. by setting the reference level for every second sensormodule to a level different from the reference level for the modulesbetween them, thus making it possible to obtain information at two oreven more different energy resolution levels about the same point in theobject by the TDI method. Such information can be utilized for moredetailed analysis of the object imaged.

[0034] By appropriately setting the reference level, it will also beeasy to elimimate the so-called postluminescence described above, orpartial propagation of the signal into the area of neighboring pixels,because the signal strength of these phenomena is of a marginal order ascompared with the instantaneously detected signal, thus remaining belowthe threshold level of the signal to be detected. In this way,compensation of the phenomena problematic in state-of-the-art solutionsis possible without the use of complicated computer algorithms.

[0035] It is naturally possible to connect to each pixel two or moreelectronic detection circuits e.g. as illustrated in FIG. 2 in parallel,in which case e.g. two reference levels can be set for each pixel. Viasuch an arrangement, it is again possible to produce images consistentwith two different minimum energy levels by a single imaging operationto allow a more detailed analysis of the object imaged.

[0036] As a detail regarding the implementation of the sensor, let it bementioned that the reference energy level for the quanta to be detectedwhich is needed in the electronic circuit for each pixel is delivered tothe circuit in the form of a current signal, which is only convertedinto a voltage level at the pixel in question or locally in the area ofa few pixels. Thus, the accuracy of the analog voltage level will not beaffected by any ground potential disturbances that may arise from otheractivities of the circuit.

[0037] If desired, it is also possible to connect to the end of eachreadout row an adder circuit which sums a desired number of numericpixel values before their being loaded into the shift register. If anidentical adder is also connected to the output end of the shiftregister to sum a desired number of numeric pixel values that can beread out, the sensor circuit can be used to implement binning, i.e.combination of pixels in X and Y directions to form larger pixels.

[0038]FIG. 5a presents a practical sensor module solution for forming aTDI sensor 50 applicable for use in scanning imaging. The sensor 50 mayconsist of e.g. four sensor module columns 51, 52, 53, 54 arrangedsuccessively in the scanning direction. In each column, individualsensor modules 510, 510′, . . . are placed in somewhat differentpositions in the transverse direction relative to the scanning movementso that any junctures in the sensor surfaces in each column are set atslightly different heights. This ensures that any gaps that may appearbetween modules will still be imaged via the other three sensor columnsand no gaps are left in the image produced. The overlapping can beimplemented e.g. using a multiple of the pixel size of the sensor modulewith the addition of a proportion of the pixel size depending on thenumber of sensor modules engaged in the image forming process in thescanning direction, said proportion being determined by the formulad_(pix)×(n+1/m), where d_(pix)=diameter of pixel, n=integer and m=numberof modules in the relevant direction or an integer smaller than this,the imaging resolution of the sensor being thus increased by signalprocessing functions beyond the physical pixel size of the sensormodule.

[0039] Corresponding overlaps and distances between modules can also beimplemented between sensor modules arranged successively in the scanningdirection, thus correspondingly increasing the resolution in thescanning direction as well. On the other hand, in the scanningdirection, a corresponding effect can also be achieved by a methodobvious to a person skilled in the art, by clocking different sensormodules in a corresponding manner.

[0040] E.g. in mammography applications, an individual module couldconsist of 142×284 pixels having a size of 35 μm, forming a sensorsurface having an area of 5 mm×10 mm, so the number of such modulescomprised in the sensor array as a whole could be four in the widthwisedirection and about 20 in elevation to form a sensor having a width ofabout 20 mm and a height of about 180 mm or 240 mm.

[0041] The gaps between sensor modules should be kept as small aspossible, on the one hand in consideration of the physical dimensions ofthe sensor arrangement, and on the other hand to allow the imaging timeneeded for carrying out the scanning movement to be kept as short aspossible in order to avoid any problems that might arise as a result ofnon-uniform production of radiation by the radiation source or inconsequence of the object being imaged moving during the imagingscanning action. In regard of the actual generation of a continuousimage, the distance between the modules is not a critical factor. Forinstance, in accordance with the above description referring to FIG. 4,a shift register can be placed at one of the vertical edges of eachsensor module without the space occupied by it causing any essentialimpediment to the imaging process. However, it is advisable to protectsuch a shift register against X-radiation e.g. by covering it with aprotective layer of the same absorbing material as is used to protectthe reading electronics, from which layer any charges appearing in thearea of the shift register can be discharged.

[0042]FIG. 5b visualizes how each module 510, 510′, . . . can be placedin a position substantially perpendicular to the focus of the ray beam12 used in the imaging process.

[0043] When such a structure consisting of a number of separate sensormodule columns arranged one after the other in the scanning direction isused, especially in TDI imaging, the scanning speed or other parametersrelating to exposure can be adjusted on the basis of the signal obtainedfrom the module column moving foremost in the scanning direction so thatthe sensors in the subsequent module columns will have an optimalexposure.

[0044]FIG. 6 illustrates a possible arrangement of overlapping sensormodules, with sensor modules 10, 10′, . . . as described above combinedto form a larger continuous image area, as seen from the focus of theradiation source, without any gaps left between modules.

[0045] As explained above, the electronics used in the invention allowsthe reference energy level for the detected quanta to be set to adesired magnitude. This can be implemented e.g. using a detectioncircuit 70 as presented in FIG. 7, by keeping the input voltagethreshold V for each pixel constant and adjusting the input impedance 71or gain 21 of the input circuit of the detection electronics for eachpixel correspondingly.

[0046] The detection technology based on the number of quanta as used inthe invention further differs to its advantage from traditional systemsmeasuring the electron charge accumulated on pixels on account of itsperfect linearity as it does not require the use of any analog signalamplifiers, in which the amplification factor typically falls as thesignal strength increases. In the technology of the invention, thecontrast resolution of image information is the same regardless ofsignal strength.

[0047] A preferred area of application of the invention is mammography,where typically the number of quanta accumulated on a 35 μm pixel is8000 quanta/s and at its maximum, without an object in between, about200 000 quanta/s. To have the quanta separated from each other, i.e. tohave them counted each one separately with a sufficient accuracy despitetheir stochastic occurrence, the reading electronics should have afrequency response of the order of a few MHz, as is customary even inmodern technology.

[0048] In the use of the technology of the invention for TDI imaging, acentral factor is the rate at which the information is read out. Inmammography imaging, a distance of 240 mm is typically scanned in threeseconds, which means that the system has to be able to read the 35 μmpixel rows at least at intervals of 430 μs. If the sensor is soimplemented that it comprises e.g. 142 columns and 284 rows and if12-bit counters are used, then within this time the system has to read284×12 bits (=3408), which would mean a read frequency of 8 MHz.Correspondingly, reading 16-bit information would require a 10.5-MHzread frequency. Such frequencies are also normal in current technology.

[0049] To maximize the resolution, when the technology of the inventionis used, the counting action of the counters should be stopped for thetime it takes to transfer their information into the counters of thenext column, because otherwise quanta that are read during the transfermight incidentally be counted into one or the other of the counters forthe two pixels. To avoid losing any quanta during this time, thetransfer time should be sufficiently short. For instance, at a normal 10MHz rate, the amount of quanta lost would be about 2% in the case of themaximum radiation rate described above, and 0.1% in a typical case ofimaging. As current technology permits the use of rates multiple timeshigher than this, the number of quanta that may be lost during thetransfer will be practically insignificant.

[0050] Although the invention has been described above by way of examplein the first place with reference to mammography applications, it cannaturally also be used in connection with any other correspondingimaging application. On the other hand, it is obvious to a personskilled in the art that, especially with the progress of technologicaldevelopment, the fundamental idea of the invention can be implemented inmay different ways and that its embodiments are not limited to theexamples described above but may instead be varied within the scope ofprotection defined in the following claims.

1. Sensor arrangement in digital X-ray imaging, said arrangementcomprising an element which absorbs X-radiation and contains a mediumconverting X-ray quanta into electron-hole pairs, in which element thesurface opposite to the surface receiving radiation is provided withelectrodes for dividing the sensor element into at least two pixelcolumns, and said arrangement comprising means for applying an electricfield across the medium material for passing, while avoiding lateraldiffusion, the charges generated during absorption to the nearestelectrode forming a pixel, each pixel electrode being provided withdetection electronics and a counter for counting the voltage or currentpulses generated by said electron-hole pairs, characterized in that thecounters for adjacent pixel columns are connected to each other so thatthe counters for the pixels can be loaded from the counter for thecorresponding pixel on the same row in the preceding pixel column. 2.Arrangement as defined in claim 1, characterized in that the function ofreading the values of the counters in the last column is implemented asan autonomous function independent of the other counters.
 3. Sensorarrangement as defined in claim 1 or 2, characterized in that thedetection electronics is implemented as part of reading electronicscomprising a silicon-based substrate.
 4. Sensor arrangement as definedin any one of claims 1-3, characterized in that the detectionelectronics comprises means for adjusting or setting a minimum energylevel for the pulses to be detected.
 5. Sensor arrangement as defined inclaim 4, characterized in that the detection electronics comprises apixel electrode, an amplifier and a comparator.
 6. Sensor arrangement asdefined in any one of claims 1-5, characterized in that there are twocircuits, each containing detection electronics and a counter, connectedto the pixel electrode.
 7. Sensor arrangement as defined in claim 5 or6, characterized in that said means for adjusting or setting the minimumenergy level for the pulses to be detected comprise means for keepingthe input voltage threshold of the comparator comprised in the detectionelectronics constant and means for adjusting the input impedance or gainof its input circuit.
 8. Sensor arrangement as defined in any one ofclaims 4-7, characterized in that it comprises possibly externallyadjustable means for setting different reference levels for differentpixels.
 9. Sensor arrangement as defined in any one of claims 4-8,characterized in that it comprises means for delivering a signaldefining the minimum energy level as a current signal and means forconverting said signal at the pixel or locally in the area of a fewpixels into a voltage level.
 10. Sensor arrangement as defined in anyone of claims 1-9, characterized in that it comprises a shift registerdisposed at an edge of said silicon-based silicon substrate element, inparticular at its trailing edge in the case of a sensor arrangement usedfor scanning imaging.
 11. Sensor arrangement as defined in claim 10,characterized in that it comprises an adder circuit placed between thepixel of each outermost pixel column and the shift register and/or atthe output end of the shift register.
 12. Sensor arrangement as definedin claim 10 or 11, characterized in that, to protect the shift registeragainst X-radiation, it is covered with radiation absorbing material.13. Sensor arrangement as defined in any one of claims 1-12,characterized in that it consists of at least two sensor modules. 14.Sensor arrangement as defined in claim 13, characterized in that thesensor modules are connected in series so that information to the inputend of the shift register of every subsequent sensor module can beloaded from the previous, preferably identical module comprised in thesensor arrangement.
 15. Sensor arrangement as defined in claim 13 or 14,characterized in that the sensor modules are arranged in a configurationwith modules overlapping in one or two directions, e.g. so that thedistance between modules equals pixel size×(n+1/m), where n=integer andm=number of modules in relevant direction or an integer smaller thanthis.
 16. Sensor arrangement as defined in any one of claims 13-15,characterized in that the sensor modules are arranged in an overlappingconfiguration so that they cover a continuous image area as seen fromthe focus of the radiation source.
 17. Sensor arrangement as defined inany one of claims 13-16, characterized in that the modules are arrangedin a number of columns and that a different reference level for thesignal to be detected has been set for the pixels in every secondcolumn.
 18. Sensor arrangement as defined in any one of claims 13-17,characterized in that it comprises means for adjusting one or moreparameters relating to exposure on the basis of the signal received fromthe first sensor module column.
 19. Method in digital X-ray imaging, inwhich method X-radiation is absorbed into a medium converting X-rayquanta into electron-hole pairs, with an element containing said mediumand having its surface opposite to the surface receiving radiationprovided with electrodes for dividing it into at least two pixelcolumns, and in which method an electric field is applied across thesaid element for directing, while avoiding lateral diffusion, thecharges generated during absorption to the nearest electrode forming apixel and further to a counter for counting the voltage or currentpulses generated by said electron-hole pairs, characterized in that thesignal read by the counters is loaded, from column to column, into thecounters for corresponding pixels on the same rows in the next pixelcolumn.
 20. Method as defined in claim 19, characterized in that thevalues of the counters in the last column are read out while the processof imaging and counting of quanta is going on.
 21. Method as defined inclaim 19 or 20, characterized in that the signal is read out via a shiftregister placed beside the last pixel column.
 22. Method as defined inany one of claims 19-21, characterized in that only pulses exceeding aset reference level are passed to the counter.
 23. Method as defined inclaim 22, characterized in that the pulse to be passed to the countersis detected by a circuit comprising a pixel electrode, an amplifier anda comparator.
 24. Method as defined in claim 22 or 23, characterized inthat the minimum energy level of the pulses to be passed to the counteris controlled by keeping the input voltage threshold of the comparatorconnected to the counter constant and adjusting the input impedance orgain of the input circuit of the comparator electronics.
 25. Method asdefined in any one of claims 22-24, characterized in that a referenceenergy level for quanta to be detected is delivered as a current signalwhich is converted at the pixel or locally in the area of a few pixelsinto a voltage level.
 26. Method as defined in any one of claims 19-25,characterized in that when the counters are being loaded, a zero valueis loaded into the counters for the pixels in the first column. 27.Method as defined in any one of claims 21-26, characterized in that thesignal from the counters in the pixel rows is summed before being passedto the shift register and/or that the signal is summed while being readout from the shift register.
 28. Method as defined in any one of claims19-27, characterized in that the counters are loaded either during animaging scanning movement or after a full-field imaging process. 29.Method as defined in any one of claims 19-28, characterized in that theimaging is performed using at least two sensor modules.
 30. Method asdefined in claim 29, characterized in that the signal obtained from agiven module is read out via the shift register of at least one othermodule.
 31. Method as defined in claim 29 or 30, characterized in thatthe reference level for the signal to be detected is adjusted to adesired value separately for each pixel, pixel column or sensor modulecolumn.
 32. Method as defined in any one of claims 29-31, characterizedin that at least two sensor module columns are used and the signal isread out at least from the first column, said signal obtained from thefirst column being used for the adjustment of one or more parametersrelating to exposure.